Why are pregnant women at risk for pyelonephritis?
Women with asymptomatic bacteriuria identified in early pregnancy who are not treated have a 20% to 30% risk of developing pyelonephritis in later pregnancy. The high risk of pyelonephritis is attributed to urine stasis from hormone-induced dilation of the smooth muscle of the urinary tract and, later in pregnancy, ureteric obstruction by pressure of the fetal head at the pelvic brim. Pyelonephritis, as with any febrile illness in later pregnancy, may lead to premature labor and delivery with adverse fetal outcomes.